U.S. patent number 10,422,892 [Application Number 14/647,141] was granted by the patent office on 2019-09-24 for photon counting x-ray detector.
This patent grant is currently assigned to KONINKLIJKE PHILIPS N.V.. The grantee listed for this patent is KONINKLIJKE PHILIPS N.V.. Invention is credited to Johan Hendrik Klootwijk, Antonius Johannes Maria Nellissen, Frank Verbakel, Herfried Karl Wieczorek.
United States Patent |
10,422,892 |
Nellissen , et al. |
September 24, 2019 |
Photon counting X-ray detector
Abstract
The present invention relates to a photon counting X-ray
detector and detection method that effectively suppress
polarization even under high flux conditions. The proposed detector
comprises a photon counting semiconductor element (10) for
generating electron-hole pairs in response to incident X-ray
photons, a cathode electrode (11a, 11b; 21a, 21b; 31a, 31b, 31c,
31ac, 31d; 41a, 41b; 51a, 51b) arranged on a first side (10a) of
said semiconductor element (10) facing incited X-ray radiation,
said cathode electrode comprising two interdigitated cathode
elements (11a, 11b; 21a, 21b; 31a, 31b, 31c, 31ac, 31d; 41a, 41b;
1a, 51b), a pixelated anode electrode (12) arranged on a second
side (10b) of said semiconductor element (10) opposite said first
side (10a), a power source (13) for applying a bias voltage between
said cathode electrode and said anode electrode and for temporarily
applying an injection voltage between said cathode elements (11a,
11b; 21a, 21b; 31a, 31b, 31c, 31ac, 31d; 41a, 41b; 51a, 51b), and a
readout unit (14) for reading out electrical signals from said
pixelated anode electrode (12).
Inventors: |
Nellissen; Antonius Johannes
Maria (Horst, NL), Verbakel; Frank (Helmond,
NL), Klootwijk; Johan Hendrik (Eindhoven,
NL), Wieczorek; Herfried Karl (Aachen,
DE) |
Applicant: |
Name |
City |
State |
Country |
Type |
KONINKLIJKE PHILIPS N.V. |
Eindhoven |
N/A |
NL |
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|
Assignee: |
KONINKLIJKE PHILIPS N.V.
(Eindhoven, NL)
|
Family
ID: |
49989864 |
Appl.
No.: |
14/647,141 |
Filed: |
November 20, 2013 |
PCT
Filed: |
November 20, 2013 |
PCT No.: |
PCT/IB2013/060267 |
371(c)(1),(2),(4) Date: |
May 26, 2015 |
PCT
Pub. No.: |
WO2014/087290 |
PCT
Pub. Date: |
June 12, 2014 |
Prior Publication Data
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Document
Identifier |
Publication Date |
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US 20150301199 A1 |
Oct 22, 2015 |
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Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
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61733006 |
Dec 4, 2012 |
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Current U.S.
Class: |
1/1 |
Current CPC
Class: |
G01T
1/241 (20130101); G01T 1/24 (20130101); G01T
1/18 (20130101); G01T 1/366 (20130101); G01T
1/247 (20130101) |
Current International
Class: |
G01T
1/24 (20060101); G01T 1/18 (20060101); G01T
1/36 (20060101) |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
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102011003246 |
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Jan 2011 |
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DE |
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102011003246 |
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Aug 2012 |
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DE |
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102011003246 |
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Aug 2012 |
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DE |
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102011003246 |
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Feb 2013 |
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DE |
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102011003246 |
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Feb 2013 |
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DE |
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09043357 |
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Feb 1997 |
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JP |
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10056196 |
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Feb 1998 |
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JP |
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Other References
Bale, D. S., et al.; Nature of polarization in wide-bandgap
semiconductor detectors under high-flux irradiation: Application to
semi-insulating Cd1--xZnxTe; 2008; Physical Review;
B77:035205-1-16. cited by applicant .
Belas, E., et al.; Electromigration of Mobile Defects in CdTe;
2009; IEEE Trans. on Nuclear Science; 56(4)1752-1757. cited by
applicant .
Del Sordo, S., et al.; Progress in the Development of CdTe and
CdZnTe Semiconductor Radiation Detectors for Astrophysical and
Medical Applications; 2009; Sensors; 9(5)3491-3526. cited by
applicant .
Grill, R., et al.; Polarization Study of Defect Structure of CdTe
Radiation Detectors; 2011; IEEE Trans. on Nuclear Science;
58(6)3172-3181. cited by applicant.
|
Primary Examiner: Smyth; Andrew
Attorney, Agent or Firm: Liberchuk; Larry
Parent Case Text
CROSS REFERENCE TO RELATED APPLICATIONS
This application is a national filing of PCT application Serial No.
PCT/IB2013/060267, filed Nov. 20, 2013, published as WO 2014/087290
A1 on Jun. 12, 2014, which claims the benefit of U.S. provisional
application Ser. No. 61/733,006 filed Dec. 4, 2012, which is
incorporated herein by reference.
Claims
The invention claimed is:
1. A photon counting X-ray detector unit comprising: a photon
counting semiconductor element for generating electron-hole pairs
in response to incident X-ray photons; a cathode electrode arranged
on a first side of said semiconductor element facing incited X-ray
radiation, said cathode electrode comprising two interdigitated
cathode elements; and a pixelated anode electrode arranged on a
second side of said semiconductor element opposite said first side,
wherein said pixelated anode electrode is configured for being
coupled to a readout unit for reading out electrical signals from
said pixelated anode electrode, wherein said photon counting X-ray
detector unit is configured to drift charge carriers from said
cathode electrode towards said anode electrode in response to an
applied bias voltage between said anode electrode and said cathode
electrode and to temporarily inject charge carriers between said
cathode elements in response to a temporarily applied injection
voltage between said cathode elements.
2. The photon counting X-ray detector unit as claimed in claim 1,
wherein said interdigitated cathode elements each comprises a
plurality of parallel electrode stripes, wherein the electrode
stripes of said interdigitated cathode elements are alternately
arranged in parallel.
3. The photon counting X-ray detector unit as claimed in claim 2,
wherein said electrode stripes comprise sharp tips arranged along
said electrode stripes, along said electrode stripes at regular
intervals.
4. The photon counting X-ray detector unit as claimed in claim 3,
wherein said tips of adjacent electrode stripes are arranged
opposite to each other.
5. A photon counting X-ray detector comprising: a photon counting
X-ray detector, comprising: a cathode electrode arranged on a first
side of said semiconductor element facing incited X-ray radiation,
said cathode electrode comprising two interdigitated cathode
elements; and a pixelated anode electrode arranged on a second side
of said semiconductor element opposite said first side; a power
source for applying a bias voltage between said cathode electrode
and said anode electrode and for temporarily applying an injection
voltage carriers between said cathode elements, and a readout unit
for reading out electrical signals from said pixelated anode
electrode, wherein said photon counting X-ray detector is
configured to drift charge carriers from said cathode electrode
towards said anode electrode in response to the applied bias
voltage between said anode electrode and said cathode electrode and
to temporarily inject charge carriers between said cathode elements
in response to the temporarily applied injection voltage between
said cathode elements.
6. The photon counting X-ray detector as claimed in claim 5,
wherein said power source is configured to temporarily apply
intermittent voltage pulses or continuous voltage wave signals
between said cathode elements.
7. The photon counting X-ray detector as claimed in claim 5,
further comprising a control unit for controlling the application
of the temporarily applied injection voltage by said power source
for controlling a pulse time, a shape, a duty cycle, a repetition
frequency and/or a voltage amplitude of intermittent voltage pulses
applied as injection voltage.
8. The photon counting X-ray detector as claimed in claim 6,
wherein said control unit is configured to control the pulse time,
the shape, the duty cycle, the repetition frequency and/or the
voltage amplitude of the temporarily applied injection voltage
based on time-of-flight drift time measurements of electrons moving
from said cathode electrode to said anode electrode.
9. The photon counting X-ray detector as claimed in claim 6,
wherein said control unit is configured to control said power
source and said readout unit to synchronize the application of the
temporarily applied injection voltage by said power source and the
readout of electrical signal from said pixelated anode electrode by
disabling the readout unit during the application of a temporarily
applied injection voltage.
10. The photon counting X-ray detector as claimed in claim 5,
wherein said power source comprises an induction unit comprising a
primary coil coupled between said two interdigitated cathode
elements and a secondary coil; a DC voltage source for applying
said bias voltage to said primary coil; and a current source for
temporarily applying intermittent current signals, including
current pulses, to said secondary coil to generate said temporarily
applied injection voltage across said primary coil.
11. The photon counting X-ray detector as claimed in claim 5,
wherein said power source is configured to temporarily apply said
injection voltage between said cathode elements with alternating
polarity.
12. The photon counting X-ray detector as claimed in claim 5,
wherein said power source is configured to apply an injection
voltage after a predetermined maximum time.
13. The photon counting X-ray detector as claimed in claim 5,
wherein said cathode electrode comprises a plurality of cathode
elements, wherein two cathode elements are interdigitated
respectively, and wherein said power source is configured to
selectively temporarily apply an injection voltage to pairs of
interdigitated cathode elements.
14. The photon counting X-ray detector as claimed in claim 5,
wherein said readout unit is configured to correct read out
electrical signals depending on the parameters of the temporarily
applied injection voltage depending on the timing and duration of
temporarily applied injection voltage.
15. A photon counting X-ray detection method comprising: subjecting
a photon counting X-ray detector unit to incident X-ray radiation
leading to the generation of electron-hole pairs in response to
incident X-ray photons, said photon counting X-ray detector unit
comprising a photon counting semiconductor element for generating
electron-hole pairs in response to incident X-ray photons; and a
cathode electrode arranged on a first side of said semiconductor
element facing incited X-ray radiation, said cathode electrode
comprising two interdigitated cathode elements; and a pixelated
anode electrode arranged on a second side of said semiconductor
element opposite said first side; applying a bias voltage between
said cathode electrode and said anode electrode; temporarily
applying an injection voltage between said cathode elements; and
reading out electrical signals from said pixelated anode electrode.
Description
FIELD OF THE INVENTION
The present invention relates to a photon counting X-ray detector,
a photon counting X-ray detector unit and a photon counting X-ray
detection method.
BACKGROUND OF THE INVENTION
Photon counting X-ray detectors (also called direct conversion
X-ray detectors) are widely known in the art and are e.g. widely
used in CT (Computed Tomography) scanners. Cadmium Telluride (CdTe)
and cadmium zinc telluride (CZT) are wide band gap semiconductor
materials that are well suited for manufacturing of (high flux)
X-ray detectors for astrophysical and medical applications (see
e.g. Stefano Del Sordo, Leonardo Abbene, Ezio Caroli, Anna Maria
Mancini, Andrea Zappettini and Pietro Ubertini: Progress in the
Development of CdTe and CdZnTe Semiconductor Radiation Detectors
for Astrophysical and Medical Applications, Sensors 2009, 9,
3491-3526). These types of detectors are very important in
applications like solid-state nuclear medicine systems and spectral
CT. These applications are based on single photon X-ray counting.
The performance of the detector is largely determined by the
quality of the crystals (mono crystalline, composition, doping
concentration, defect density) and the materials and processes used
to form the electrodes on the detector (barrier height of the used
metal with respect to semiconductor, contact resistance, sheet
resistance, adhesion, etc.). Also mechanical processing and surface
preparation (dicing, grinding, polishing and cleaning) and
eventually the passivation have a large influence on the final
performance.
The performance of CdTe and CZT detectors is often critically
disturbed by charging of the bulk material of the detector, which
causes local build-up of an internal electric field and counteracts
the applied bias voltage. This effect is known as polarization of
the detector. Polarisation especially occurs under high flux X-ray
exposure conditions and strongly limits the performance of spectral
CT photon counting.
U.S. Pat. No. 5,821,539 discloses a direct converting radiation
detector with a diode-like (or sandwich-like) structure having
first and second operating electrodes on opposite sides of a
semiconductor body having an additional injector electrode, which
injects charge carriers for the compensating charged traps in the
semiconductor body. The secondary dark current generated in this
way does not (or minimally) flow via the electrode used for
measurement purposes and therefore does not influence the measured
signal. The injection is facilitated by suitable doping under the
injector electrode.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide to a photon
counting X-ray detector, a photon counting X-ray detector unit and
a photon counting X-ray detection method that enable suppression of
polarization under high flux conditions.
In a first aspect of the present invention a photon counting X-ray
detector is presented that comprises: a photon counting
semiconductor element for generating electron-hole pairs in
response to incident X-ray photons, a cathode electrode arranged on
a first side of said semiconductor element facing incited X-ray
radiation, said cathode electrode comprising two interdigitated
cathode elements, a pixelated anode electrode arranged on a second
side of said semiconductor element opposite said first side, a
power source for applying a bias voltage between said cathode
electrode and said anode electrode and for temporarily applying an
injection voltage between said cathode elements, and a readout unit
for reading out electrical signals from said pixelated anode
electrode.
In a further aspect of the present invention a photon counting
X-ray detector unit is presented that comprises: a photon counting
semiconductor element for generating electron-hole pairs in
response to incident X-ray photons, a cathode electrode arranged on
a first side of said semiconductor element facing incited X-ray
radiation, said cathode electrode comprising two interdigitated
cathode elements, said cathode electrode being configured for being
coupled to a power source for applying a bias voltage between said
cathode electrode and said anode electrode and for temporarily
applying an injection voltage between said cathode elements, and a
pixelated anode electrode arranged on a second side of said
semiconductor element opposite said first side, wherein said
pixelated anode electrode is configured for being coupled to a
readout unit for reading out electrical signals from said pixelated
anode electrode, wherein said cathode electrode is configured for
being coupled to a power source for applying a bias voltage between
said cathode electrode and said anode electrode and for temporarily
applying an injection voltage between said cathode elements.
In yet a further aspect of the present invention there is provided
a photon counting X-ray detection method that comprises: subjecting
a photon counting X-ray detector unit to incident X-ray radiation
leading to the generation of electron-hole pairs in response to
incident X-ray photons, said photon counting X-ray detector unit
comprising a photon counting semiconductor element for generating
electron-hole pairs in response to incident X-ray photons, a
cathode electrode arranged on a first side of said semiconductor
element facing incited X-ray radiation, said cathode electrode
comprising two interdigitated cathode elements, and a pixelated
anode electrode arranged on a second side of said semiconductor
element opposite said first side, applying a bias voltage between
said cathode electrode and said anode electrode temporarily
applying an injection voltage between said cathode elements, and
reading out electrical signals from said pixelated anode
electrode.
Preferred embodiments of the invention are defined in the dependent
claims. It shall be understood that the claimed detection method
and detector unit have similar and/or identical preferred
embodiments as the claimed detector and as defined in the dependent
claims.
To suppress polarization of the detector under high flux conditions
a special configuration of the cathode electrode, which allows
temporal injection of electrons and thereby neutralisation of hole
traps that cause polarisation, is proposed according to the present
invention. In particular, the proposed cathode electrode comprises
two interdigitated cathode elements at which, during short
intervals, an injection voltage (e.g. intermittent voltage pulses
or a continuous (pulse-like) voltage wave signal) is applied. Said
injection voltage has the effect that electrons are temporarily
injected into the photon counting semiconductor element. These
electrons are moving to the anode electrode due to the applied bias
voltage between the cathode electrode and the anode electrode and
thus can neutralize occupied hole traps within the photon counting
semiconductor element that caused said polarization.
In an embodiment said detector further comprises a control unit for
controlling the application of the injection voltage by said power
source. Thus, the various parameters of the injection voltage, in
particular the intermittent voltage pulses, can be controlled to
optimally achieve polarization suppression.
Accordingly, in an embodiment said control unit is configured to
control pulse time, shape, duty cycle, repetition frequency and/or
voltage amplitude (or any other parameter) of the temporarily
applied injection voltage, in particular of intermittent voltage
pulses, i.e. to control one or more parameters that have an
influence on the polarization suppression and which may also depend
on other parameters like the layout and dimensions of the detector,
the patterns of the electrodes, the applied voltages, etc.
Further, said control unit is preferably configured to control
pulse time, shape, duty cycle, repetition frequency and/or voltage
amplitude of the temporarily applied injection voltage based on
time-of-flight drift time measurements of electrons moving from
said cathode electrode to said anode electrode. Said time-of-flight
drift time measurements indicate the time required by electrons to
drift from the cathode electrode to the anode electrode and give an
indication of the effectiveness of the polarization
suppression.
According to still another embodiment said control unit is
configured to control said power source and said readout unit to
synchronize the application of the temporarily applied injection
voltage by said power source and the readout of electrical signal
from said pixelated anode electrode such that during the
application of the injection voltage no electrical signals are read
out from said pixelated anode electrode. Thus, the injected
electrons are not counted and the counting result is not
falsified.
There are many layouts usable for the arrangement of the
interdigitated cathode elements. In a preferred arrangement, said
interdigitated cathode elements each comprises a plurality of
parallel electrode stripes, wherein the electrode stripes of said
interdigitated cathode elements are alternately arranged in
parallel. This provides for a good injection of electrons into the
semiconductor element. Alternative layouts are e.g. based on a
fishbone or spiral structure or a combination of the mentioned
structures. Preferably, the interdigitated cathode elements are
aligned with respect to the anode electrode which might improve
homogeneity between samples, i.e. of injected electrons per
pixel.
An even further improvement, in particular due to higher local
electrical fields, is achieved by an embodiment in which said
electrode stripes comprise sharp tips arranged along said electrode
stripes. Preferably, said tips are arranged along said electrode
stripes at regular intervals and/or said tips of adjacent electrode
stripes are arranged opposite to each other, which further improve
the injection of electrons.
In an advantageous embodiment said power source comprises an
induction unit comprising a primary coil coupled between said two
interdigitated cathode elements and a secondary coil, a DC voltage
source for applying said bias voltage to said primary coil, and a
current source for applying intermittent signals, in particular
current pulses, to said secondary coil to generate said temporarily
applied injection voltage across said primary coil. This provides
for a simple way of applying the required voltages to the cathode
elements and the anode. With this set-up it is generally easier to
keep the average main electric field between anode electrode and
cathode electrode constant. Applying separate voltage sources is
alternatively possible, but may require measures to guarantee that
the average main electric field between anode electrode and cathode
electrode constant.
Preferably, said power source is configured to temporarily apply
said injection voltage between said cathode elements with
alternating polarity. This avoids local polarization within the
semiconductor element.
In another embodiment said power source is configured to apply an
injection voltage after a predetermined maximum time. Thus, after
said maximum time a reset of the semiconductor material is made by
injecting electrons to neutralize all hole traps that have been
formed so far.
Preferably, said cathode electrode comprises a plurality of cathode
elements, wherein two cathode elements are interdigitated
respectively, and wherein said power source is configured to
selectively temporarily apply an injection voltage to said pairs of
interdigitated cathode elements. Thus, the injection of electrons
to different portions of the detector can be individually
controlled for the different portions by controlling the injection
voltage applied to the pairs of interdigitated cathode elements
covering said different portions.
Still further, in an embodiment said readout unit is configured to
correct read out electrical signals depending on the parameters of
temporarily applied injection voltage, in particular depending on
the timing and duration of the temporarily applied injection
voltage. Thus, the accuracy of the obtained photon counting result
is increased. In still another embodiment a feedback loop is
provided that reads polarization and then only give a voltage pulse
when needed.
BRIEF DESCRIPTION OF THE DRAWINGS
These and other aspects of the invention will be apparent from and
elucidated with reference to the embodiment(s) described
hereinafter. In the following drawings
FIG. 1 shows a schematic diagram of a first embodiment of a photon
counting X-ray detector according to the present invention,
FIG. 2 shows a top view of a first embodiment of a cathode
electrode according to the present invention,
FIG. 3 shows a schematic diagram of a second embodiment of a photon
counting X-ray detector according to the present invention,
FIG. 4 shows a top view of a second embodiment of a cathode
electrode according to the present invention,
FIG. 5 shows a top view of a third embodiment of a cathode
electrode according to the present invention,
FIG. 6 shows a top view of a third embodiment of a cathode
electrode according to the present invention,
FIG. 7 shows a top view of a fourth embodiment of a cathode
electrode according to the present invention,
FIG. 8 shows a top view of a fifth embodiment of a cathode
electrode according to the present invention, and
FIG. 9 shows a flowchart of a photon counting method according to
the present invention.
DETAILED DESCRIPTION OF THE INVENTION
FIG. 1 shows a schematic diagram of a first embodiment of a photon
counting X-ray detector 1 according to the present invention. It
comprises a photon counting semiconductor element 10 for generating
electron-hole pairs in response to incident X-ray photons 2, a
cathode electrode 11 arranged on a first side 10a of said
semiconductor element 10 facing incident X-ray radiation 2 and a
pixelated anode electrode 12 arranged on a second side 10b of said
semiconductor element 10 opposite said first side 10a.
As clearly shown in the top view of the cathode electrode 11
depicted in FIG. 2 the cathode electrode 11 comprises two
interdigitated cathode elements 11a, 11b. Said interdigitated
cathode elements 11a, 11b each comprise a plurality of parallel
electrode stripes 11c, 11d which are alternately arranged in
parallel.
The semiconductor element 10, the cathode electrode 11 and the
anode electrode 12 thus form a photon counting X-ray detector unit
3 which may be used with various electronics for providing voltage
and/or current signals and for reading out signals.
The photon counting X-ray detector 1 further comprises a power
source 13 for applying a bias (DC) voltage between said cathode
electrode 11 and said anode electrode 12 and for temporarily
applying an injection voltage between said cathode elements 11a,
11b. Still further, a readout unit 14 is provided for reading out
electrical signals from said pixelated anode electrode 12.
In the following explanation as example for the injection voltage
intermittent (temporary) voltage pulses will be considered. In
practice, voltage wave signals can alternatively used as
temporarily applied injection voltage.
In a preferred embodiment of the X-ray detector 1' shown in FIG. 3
said semiconductor element 10 comprises a CdZnTe detector crystal
(also called CZT detector crystal). For both the interdigitated
electrode pattern 11 at the cathode side and the pixelated
electrode pattern 12 at the anode side a metal is preferably chosen
with a high potential barrier towards CdZnTe causing blocking
contacts. For instance, for n-type CZT platinum and for p-type CZT
(and CdTe) indium or aluminum is used as blocking (Schottky)
contact.
In this embodiment the power unit comprises an induction unit 130
comprising a primary coil 131 coupled between said two
interdigitated cathode elements 11a, 11b and a secondary coil 132.
A DC voltage source 133 is provided for applying said bias voltage
to said primary coil 131, and a current source 134 is provided for
applying intermittent current pulses to said secondary coil 132 to
generate said intermittent voltage pulses across said primary coil
131. Thus, the cathode electrode 11 is connected via the induction
coil 131 to a bias voltage in the range from -0.1 kV to -10 kV,
e.g. -1 kV, and the anode pixels of the anode electrode 12 are
connected to the readout chip 14. In this way an electric field is
created over the detector crystal 10 in the order of 100-500 V/mm,
preferably 200-400, e.g. 300 V/mm.
Generally, in a photon counting X-ray detector an absorbed X-ray
photon generates a cloud of electron-hole pairs in the
semiconductor element 10. Due to the existence of the electric
field the electrons move towards the anode electrode 12 and are
collected by the readout unit 14. In this way energy, location and
timing of each incident photon is registered. The generated holes
move towards the cathode electrode 11. Holes have a much lower
mobility than electrons and can be trapped easily in hole traps.
This causes polarization of the detector and occurs especially
under high flux X-ray exposure conditions. Polarization may lead to
complete failure of the photon counting method.
In order to suppress polarization the trapped holes must be
neutralized. This is achieved according to the present invention by
temporal injection of electrons at the cathode electrode 11. Under
normal operation conditions the cathode contact is blocking,
resulting in a low dark current level, which is very advantageous.
Temporal injection of electrons is initiated by a short voltage
pulse between said cathode elements 11a, 11b, caused e.g. by a
short current pulse through the secondary coil 132 of the induction
unit 131 in the second embodiment of the X-ray detector 1' shown in
FIG. 3. This current pulse induces a high voltage difference
(field) between the two adjacent stripes 11c, 11d (also called
legs) of the interdigitated cathode electrode 11, which will force
electrons to move into the semiconductor element 10 even though
blocking contacts are used. Once the electrons are forced into the
semiconductor element 10, they drift towards the anode electrode 12
because of the applied bias voltage and can thus neutralize
occupied hole traps (or, to be more precise, trapped holes) within
the semiconductor element 10. This in turn suppresses polarization
of the detector.
Optionally, a control unit 15 is additionally provided, as shown in
FIG. 3, for controlling the application of the intermittent voltage
(polarization suppression) pulses by said voltage source 134 (or,
generally, by the power source 13). Particularly pulse time, shape,
duty cycle, repetition frequency and/or voltage amplitude of the
intermittent voltage pulses (or any other parameter of an applied
injection voltage) as applied by said power source can thus be
controlled to optimize the suppression of polarization. Preferably,
one or more of these parameters are controlled taking into account
the given X-ray exposure dose.
Further, by use of the control unit 15 the readout unit 14 can be
synchronized with the polarization suppression pulse. Thus, care is
taken that the injected electrons are not counted. Another
advantage of this control unit is that the high anode-to-cathode DC
voltage is only slightly changed during the short pulse time. The
main electric field is hardly disturbed.
The effectiveness of polarization suppression or the degree of
polarization of the semiconductor element 10 can be derived from
time-of-flight (drift time) measurements indicating the time
necessary for electrons to move from the cathode electrode 11 to
the anode electrode 12.
There are many variations possible on the pattern layout of the
cathode pattern (i.e. layout of the cathode elements 11a, 11b) and
the materials used and on the ways to generate the temporal high
electric field to inject electrons. A variation on pattern layout
is depicted in FIG. 4 showing a top view of another embodiment of a
cathode electrode 21. This cathode electrode 21 comprises two
cathode elements 21a, 21b, wherein sharp tips 21e are attached at
regular distances along the straight legs 21c, 21d of the
interdigitated structure on the cathode pattern. This results in
even higher local electrical fields which facilitates injection of
electrons.
Preferably, in an embodiment positive and negative voltage pulses
are alternately applied at the interdigitated cathode elements 11a,
11b to avoid local (lateral) polarization and electromigration.
The timing of the voltage pulses is preferably controlled. When a
voltage pulse is only approximately 100 psec to 1 nsec (or max. 10
nsec) long, many electrons may be injected and then slowed down
under `standard` electric field conditions so that they have time
for recombination with trapped holes. This avoids a long `dead`
time for the detector.
In an embodiment a maximum time is defined after which such a
`reset` voltage pulse is applied to the cathode elements 11a, 11b.
This time may depend on illumination conditions (i.e. on the
incident radiation). For instance, a look-up table can be used to
define, e.g. depending on X-ray flux, the time for a reset. This
look-up table can be stored in the control unit 15 or in a separate
storage unit (not shown) that can be accessed by the control unit
15.
Another embodiment of a cathode electrode 31 is shown in FIG. 5. In
this embodiment the cathode electrode 31 comprises a plurality of
cathode elements 31a, 31b, 31c, 31d, wherein two cathode elements
are interdigitated respectively. For instance the cathode elements
31a, 31b are interdigitated and the cathode elements 31c, 31d are
interdigitated. The power source 13 is configured to selectively
apply intermittent voltage pulses to said pairs of interdigitated
cathode elements 31a, 31b, 31c, 31d. Thus, the control of the
application of the voltage pulses can be done differently for
different parts of the detector, e.g. for different detector
modules (each comprising e.g. a pair of cathode elements). This may
be used to avoid a reset in low-flux regions, aiming at maximum
signal there, while in high-flux regions part of the signal may be
lost during the reset phase.
There are further variations for the arrangement, pattern and
number of cathode elements that can be used in a detector according
to the present invention. For instance, in a variation of the
embodiment shown in FIG. 5, the cathode elements 31a and 31c are
combined into a common cathode element which also enables to
separately apply voltage pulses to the pairs 31ac, 31b and 31ac,
31d (31ac representing the common cathode element of cathode
elements 31a and 31c). Such a cathode electrode 31' is shown in
FIG. 7.
Still further embodiments of cathode patterns are shown in FIGS. 8
and 9. In the embodiment shown in FIG. 8 the cathode electrode 41
comprises cathode elements 41a and 41b formed as a fishbone
structure. In the embodiment shown in FIG. 9 the cathode electrode
51 comprises cathode elements 51a and 51b formed as a spiral
structure.
Generally, it is preferred that the cathode elements are aligned
with respect to the anode electrode to achieve homogeneity of the
injected electrons per pixel.
In another embodiment the readout unit 14 is configured to correct
read out electrical signals (of the anode pixels) depending on the
parameters of applied intermittent voltage pulses, in particular
depending on the timing and duration of applied intermittent
voltage pulses. In this way the accuracy of the counting result is
increased. This can e.g. be implemented by use a look-up table to
store the impact of different intermittent voltage pulses (rep.
frequency, amplitude etc.) on the read out signals (signal height,
offsets due to extra charge injected, non-linearities, etc.).
In still another embodiment the legs (or stripes) of the
interdigitated cathode elements of the cathode electrode are made
of different material. One cathode element is made of a high
barrier metal that yields a blocking contact (e.g. Pt) and the
other cathode electrode is made of a low barrier metal that yields
an ohmic contact (e.g. In). Semi-injecting is also possible (e.g.
Cr, Ag), in which case less electrons are injected. During photon
counting only the high barrier cathode electrode is connected,
which provides low dark current because of blocking contact. During
the "reset" period also the low barrier cathode electrode is
connected which causes temporal injection of electrons which
suppresses polarization. A fast switch is used to switch the low
barrier leg on and off.
An embodiment of a photon counting X-ray detection method according
to the present invention is shown as flowchart in FIG. 9. Said
method comprises the following steps. In a first step S1 a photon
counting X-ray detector unit (as e.g. depicted in FIG. 1 as
detection unit 3) is subjected to incident X-ray radiation leading
to the generation of electron-hole pairs in response to incident
X-ray photons. In a second step S2 a bias voltage is applied
between said cathode electrode and said anode electrode. In a third
step S3 voltage pulses are intermittently applied between said
cathode elements. In a fourth step S4 electrical signals read out
from said pixelated anode electrode.
With the detector, detector unit and detection method according to
the present invention suppression of polarization under high flux
conditions can be reliably and effectively obtained and the
accuracy of photon counting results can be increased.
While the invention has been illustrated and described in detail in
the drawings and foregoing description, such illustration and
description are to be considered illustrative or exemplary and not
restrictive; the invention is not limited to the disclosed
embodiments. Other variations to the disclosed embodiments can be
understood and effected by those skilled in the art in practicing
the claimed invention, from a study of the drawings, the
disclosure, and the appended claims.
In the claims, the word "comprising" does not exclude other
elements or steps, and the indefinite article "a" or "an" does not
exclude a plurality. A single element or other unit may fulfill the
functions of several items recited in the claims. The mere fact
that certain measures are recited in mutually different dependent
claims does not indicate that a combination of these measures
cannot be used to advantage.
Any reference signs in the claims should not be construed as
limiting the scope.
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